The present invention relates to heterogeneous catalysts for hydrogenation of unsaturated polymers, particularly higher molecular weight polymers, e.g., having molecular weights in the range from about 65,000 to about 500,000.
Polymers containing ethylenic and aromatic unsaturation such as styrene-isoprene-styrene and styrene-butadiene-styrene triblock copolymers have broad commercial use in adhesive formulations, footwear and as polymer modifiers. Such copolymers contain one unit of ethylenic unsaturation for each unit of diene in the final polymer. The ethylenic unsaturation provides sites for chemical reactions to occur making the polymer susceptible to thermal degradation, oxidative degradation, instability under ultraviolet radiation and general poor weatherability.
Polymers that have the ethylenically unsaturated region saturated via hydrogenation have greatly improved stability and weathering properties because the reactive site of ethylenic unsaturation is removed. Hydrogenation may also improve some of the physical properties of these polymers, see, for example, British Patent 1,229,573 and U.S. Pat. No. 3,333,024. However, hydrogenation of the aromatic unsaturation can have deleterious effects on polymer properties, such as a loss of rubber-like characteristics. Thus, it is desirable that the hydrogenation be selective to saturation of the ethylenically unsaturated region with minimal hydrogenation of the aromatic region.
Both homogeneous and heterogeneous catalyst systems have been widely used for the hydrogenation of ethylenically unsaturated polymers. Homogeneous catalytic processes are disclosed in U.S. Pat. Nos. 3,595,295; 3,595,942; 3,700,633 and 3,810,957, as well as in "Polymer Hydrogenations With Soluble Lithium/Cobalt And Aluminum/Cobalt Catalysts"; J. C. Falck, Catalysis In Organic Synthesis, E. D. PN Rylander and H. Greenfield, Academic Press, New York, 1976, pp. 305-24. Homogeneous catalysts can be highly selective with respect to effecting hydrogenation of ethylenically unsaturated moieties. These catalysts exist in the same phase as the reaction mixtures, which makes their separation from the hydrogenated polymer difficult and adds significantly to process costs.
The heterogeneous catalysts, on the other hand, exist as a phase distinct from the reaction mixture, and their separation from the hydrogenated polymer is more easily accomplished. Unfortunately, extensive hydrogenation of the aromatic region typically occurs along with hydrogenation of the ethylenically unsaturated region when heterogeneous catalyst systems are used to hydrogenate the polymer. This is shown in a number of patents and confirmed by our studies on commercial catalysts, see U.S. Pat. Nos. 3,333,024; 3,415,789; Belgium Patent BE871348 and British Patent GB 2,011,911.
Considerable research has been directed to the development of heterogeneous catalyst systems which would more selectively hydrogenate the ethylenically unsaturated region while reducing the hydrogenation of the aromatic region. One such approach is described in French Patent 2,468,618 (British equivalent GB 2,061,961). This patent describes the use of a 5% rhodium/activated carbon catalyst as a selective hydrogenation catalyst for a number of aromatic-diene copolymer systems. In particular, two examples are given of selective hydrogenation of triblock copolymers. Examples 3 and 8 of the French patent indicate selective hydrogenation with triblock copolymers whose molecular weight is 60,000. Our studies indicate that a 5% rhodium/activated carbon catalyst employing an activated carbon having an average particle size of 20-40 microns and a surface area of 1,100 square meters per gram has poor, nonselective activity for hydrogenation of a commercial styrene-butadiene-styrene block copolymer whose average molecular weight is 177,000. The poor activity is also independent of catalyst particle size with particle sizes as small as 5 microns. Although this catalyst may have some use for low molecular weight polymers, it is not useful for higher molecular weight polymers and falls outside the range of catalysts specified later in this invention.
A second approach which describes a process for the selective hydrogenation of ethylenic-aromatic unsaturated polymer systems is U.S. Pat. No. 4,501,685. This reference describes a system in which a poison is added to the feed to moderate the activity of the catalyst, thereby giving good selectivity for hydrogenation of the olefin region without hydrogenation of the aromatic region. The best results reported show that the aromatic hydrogenation is about one-half of the total hydrogenation, i.e., at 68% olefin hydrogenation, the aromatic hydrogenation is 34%. Extrapolating these results to almost quantitative conversion of the olefin would lead one to expect at least 50% hydrogenation of the aromatic region. This is not a very selective process. There is also a good possibility that some of the poison may end up in the polymer causing detrimental effects on its properties.
Another patent, U.S. Pat. No. 4,560,817, describes selective hydrogenation catalysts for ethylenically-aromatic unsaturated polymer systems. This reference also describes the use of poisons to alter the selectivity of hydrogenation catalysts. Specifically, lithium methoxide (LiOCH.sub.3) or ammonia are used as poisons to moderate the activity of the catalyst. For the reaction to be effective, the hydrogenation must be conducted at temperatures where polymer degradation occurs, e.g., 160.degree.-167.degree. C. When the hydrogenation is effected at a lower temperature, e.g., 102.degree. C., only a small amount of the unsaturation, in the range of 16%, is converted.
U.S. Pat. No. 4,452,951 discloses a process for producing hydrogenated conjugated diene polymers employing a heterogeneous catalyst comprising porous silica having a specific surface area of not more than 600 square meters per gram and an average pore diameter of from 80-1,200 angstroms as a carrier for the hydrogenation catalyst, which can be any of the metallic or nonmetallic catalysts which have hydrogenating ability. The pore volumes which are recited appear to be a mix of calculated values, as in Example 1, and measured, as in Example 4. While the reference does teach catalyst supports having average pore diameters of 1200 angstroms, it does not teach pore size distribution as an important criterion.